In the chloride process for making titanium dioxide, titanium tetrachloride is oxidized in the vapor phase to titanium dioxide. The titanium dioxide and other reaction products are then passed through an externally cooled conduit where they are cooled and coalesced. During the cooling process, it has been found that solid deposits can adhere to the internal walls of the conduit and can significantly impair the heat transfer capability of the conduit. To remedy this problem, scrubbing materials can be introduced into the cooling conduit. For example U.S. Pat. No. 2,721,626 discloses the use of calcined titanium materials. Also, U.S. Pat. Nos. 2,899,278 and 4,784,841 disclose the use of calcined titanium dioxide for scrubbing materials. If desired, the scrubbing material can aid the cooling process by introducing it into the cooling conduit at a temperature less than that of the reaction products.
A problem with the existing scrubbing materials, however, is that they can contaminate the final titanium dioxide pigment product. For example, the calcined titanium dioxide and silica sand particles are larger than the finished pigment particles and are difficult to remove. This contamination, of course, is undesirable because it can cause lumps in a paint film, or on the surface of paper, polymers or other materials in which the final titanium dioxide pigment product is incorporated. Residual calcined titanium dioxide scrubbing material and silica sand grains can also dull TiO.sub.2 processing equipment and the processing equipment of users of TiO.sub.2, for example, paper cutters and other paper processing equipment.